Through-substrate optical coupling to photonics chips

ABSTRACT

An optoelectronic integrated circuit for coupling light to or from an optical waveguide formed in an optical device layer in a near-normal angle to that layer. In an embodiment, the integrated circuit comprises a semiconductor body including a metal-dielectric stack, an optical device layer, a buried oxide layer and a semiconductor substrate arranged in series between first and second opposite sides of the semiconductor body. At least one optical waveguide is formed in the optical device layer for guiding light in a defined plane in that device layer. Diffractive coupling elements are disposed in the optical device layer to couple light from the waveguide toward the second surface of the semiconductor body at a near-normal angle to the defined plane in the optical device layer. In an embodiment, an optical fiber is positioned against the semiconductor body for receiving the light from the coupling elements.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.H90011-08-C-0102 (NOBS) (Defense Advanced Research Projects Agency(DARPA)). The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

This invention generally relates to optoelectronic integrated circuits,or photonics chips, and more specifically, to optical coupling between awaveguide and a photonics chip.

Optoelectronic integrated circuits (ICs) include both electronic andoptical elements within a single chip. Typical electronic elementsinclude field effect transistors, capacitors, and resistors; and typicaloptical elements include waveguides, optical filters, modulators, andphotodetectors. Within a given optoelectronic IC, some of the electronicelements may be dedicated to handling tasks such as data storage andsignal processing. Other electronic elements may be dedicated tocontrolling or modulating the optical elements. Including both types ofelements on a single chip provides several advantages, which includereduced layout area, cost, and operational overhead. In addition, suchintegration yields hybrid devices, such as an opto-isolator.

The integration of optical and electronic elements has been greatlyfacilitated by the maturation of today's semiconductor processingtechniques. For instance, conventional processing techniques may beadapted to create silicon-based prisms, waveguides, and other opticaldevices.

One device, however, that has been difficult to integrate is a siliconbased laser or light source. As a result, most optoelectronic ICs areadapted to receive an externally applied light beam from a laser or anoptical fiber. Unfortunately, introducing a light beam to an IC canoften be difficult. For example, in order for an optoelectronic IC toaccommodate a light beam, the spot size and the numerical aperture (NA)of the beam may need to be appropriately matched to optical elementswithin an IC.

Two primary methods for coupling standard single-mode optical fibersto/from high-index contrast photonic platforms are (1) edge couplingbased on inverse tapered waveguides, and (2) near-normal coupling basedon resonant waveguide gratings. With the former approach, the inversetapered waveguides (e.g., Si) are evanescently coupled to lower indexcontrast waveguides (e.g., SiON) with mode-field diameters larger thanthe high-index-contrast waveguides but still much smaller than those ofstandard single-mode optical fiber.

This edge coupling approach may suffer from limited real estateavailable at the chip edge for optical I/O, limited I/O density limitedto 1-D fiber arrays, and additional complexity (such as fiber tapers,lenses, etc.) required for interfacing the unmatched mode fielddiameters at the chip edge and fiber facet. The second approachalleviates many of these problems. First, it allows coupling of 2-Dfiber arrays to 2-D layouts of waveguide gratings on the chip surface.Second, it can provide fiber-matched mode-field diameters inwell-designed gratings. However, the intrinsic trade-off in the gratingcouplers between coupling efficiency and bandwidth, as well as afundamental coupling efficiency limit of less than unity, plaguessystems seeking to implement this technology. Additionally, the standardapproach of coupling from the top of the photonic device layer is notcompatible with CMOS-processing, which includes several layers of metalsand dielectrics above the photonic device layer.

In one prior art system, a metal mirror may be introduced above agrating to retransmit the uncoupled light back into the grating a secondtime for increased efficiency. In this implementation, the optical beampasses through the chip substrate, but the optical properties of thesubstrate are not leveraged for increased coupling performance. Instead,a complex lensing system is used near a remote fiber facet. Anotherprior art system implements prisms within and around the substrate tocouple light into and out of the photonic waveguides from the bottomside of the chip. However, evanescent coupling schemes, rather thangrating-based coupling schemes, are used, which require a costlythinning of the buried oxide layer to less than 250 nm (typically 1 to 3um).

BRIEF SUMMARY

Embodiments of the invention provide an optoelectronic integratedcircuit for coupling light to or from an optical waveguide formed in anoptical device layer in a near-normal angle to that layer. In anembodiment, the integrated circuit comprises a semiconductor bodyincluding a metal-dielectric stack, an optical device layer, a buriedoxide layer and a semiconductor substrate arranged in series betweenfirst and second opposite sides of the semiconductor body. At least oneoptical waveguide is formed in the optical device layer for guidinglight in a defined plane in the optical device layer. A plurality ofdiffractive coupling elements are disposed in the optical device layerto couple light from the waveguide to the second surface of thesemiconductor body at a near-normal angle to the defined plane in theoptical device layer.

In an embodiment, an array of lenses is formed in the second surface ofthe semiconductor body to receive light from or to transmit light to theplurality of diffractive coupling elements.

In an embodiment, a metallic reflective layer is formed in themetal-dielectric stack adjacent the diffractive coupling elements toredirect light to the diffractive coupling elements and therein enhanceoptical coupling efficiency.

In one embodiment, the plurality of diffractive coupling elementscomprise a grating patterned into the at least one optical waveguide toallow near-normal light reflection to and from the optical waveguide.

In an embodiment, one or more mechanical elements are fabricated on thesecond surface of the semiconductor body to facilitate positioning anexternal optical fiber or optical waveguide on the semiconductor body ina position aligned with the array of lenses.

In one embodiment, one or more grooves are formed in the second surfaceof the semiconductor body to reflect light from the diffractive couplingelements into a direction substantially parallel with the first andsecond surfaces of the semiconductor body to an edge of thesemiconductor body.

In an embodiment, at least one of said one or more grooves is a V-shapedgroove extending into the semiconductor substrate from the secondsurface of the semiconductor body and aligned with the diffractivecoupling elements.

In one embodiment, one or more mechanical elements are fabricated on thesemiconductor body to facilitate positioning an external optical fiberor optical waveguide on the semiconductor body in a position aligned toreceive the light reflected by said one or more grooves.

Embodiments of the invention provide a method of fabricating anoptoelectronic integrated circuit. In an embodiment, the methodcomprises fabricating a semiconductor body including a metal-dielectricstack, an optical device layer, a buried oxide layer and a semiconductorsubstrate arranged in series between first and second sides of thesemiconductor body. Fabricating this semiconductor body includes formingat least one optical waveguide in the optical device layer for guidinglight in a defined plane in the optical device layer, and forming aplurality of diffractive coupling elements in the optical device layerto couple light from the waveguide to the second surface of thesemiconductor body at a near-normal angle to said defined plane.

In one embodiment, an array of lenses is formed in the second surface ofthe semiconductor body to receive light from or to direct light to theplurality of diffractive coupling elements.

In an embodiment, a metallic reflective layer is formed in saidmetal-dielectric stack adjacent the diffractive coupling elements toredirect light to the diffractive coupling elements and therein enhanceoptical coupling efficiency.

In one embodiment, the plurality of diffractive coupling elements areformed by patterning a grating into the optical waveguide to allownear-normal light reflection to and from the optical waveguide.

In an embodiment, one or more mechanical elements are fabricated on thesecond surface of the semiconductor body to facilitate positioning anexternal optical fiber or optical waveguide against said third side ofthe semiconductor body in a position aligned to receive the lightreflected by said at least one groove.

Embodiments of the invention provide an optoelectronic transceivermodule. The module comprises a semiconductor body including ametal-dielectric stack, an optical device layer, a buried oxide layerand a semiconductor substrate arranged in series between first andsecond sides of the semiconductor body. At least one optical waveguideis formed in the optical device layer for guiding light in a definedplane in the optical device layer. A plurality of diffractive couplingelements are disposed in the optical device layer to direct light fromthe waveguide toward the second surface of the semiconductor body at anear-normal angle to said defined plane. A fiber array connector isassembled on the semiconductor body, and this connector comprises anarray of optical fibers arranged in a plane approximately parallel tothe defined plane in the optical device layer for receiving said lightfrom the waveguide.

In one embodiment, the fiber array connector is assembled on the secondsurface of the semiconductor body, and the fiber array connectorincludes an array of lenses for focusing the light from the waveguideonto the fiber array.

In an embodiment, the fiber array connector includes mechanicalalignment elements for positioning the fiber array connector in aspecified position on the semiconductor body.

In one embodiment, the semiconductor body includes an array of lensesformed on the second surface of the semiconductor body to receive lightfrom the plurality of diffractive elements. The mechanical alignmentelements position the fiber array connector on the semiconductor bodywith the array of lenses of the fiber array connector aligned with thearray of lenses of the semiconductor body.

In an embodiment, the semiconductor body includes at least one grooveformed in the second surface of the semiconductor body to reflect lightfrom the diffractive coupling elements into a direction substantiallyparallel with the first and second surfaces of the semiconductor body toan edge of the semiconductor body, said edge being on a third side ofthe semiconductor body, between the first and second sides thereof. Thefiber array connector comprises one or more mechanical alignmentelements to facilitate positioning the fiber array connector adjacentsaid edge of the semiconductor body in a position aligned to receive thelight reflected by said at least one groove.

Embodiments of the invention provide backside optical coupling fromoptical waveguides fabricated on the front surface of a photonic chipthrough the semiconductor substrate. Embodiment of the invention usegrating-based near-normal optical coupling schemes disposed on the frontwaveguide surface in combination with lens elements and alignmentfeatures integrated on the back surface of the substrate. In embodimentsof the invention, the addition of metal mirrors disposed over thegrating structure is used to enhance efficiency. The grating coupler isalso compatible with polarization diversity schemes, mode-matching tospecific optical fiber mode dimensions, and 2-D fiber array coupling.

Embodiments of the invention can apply to passive photonic circuits,photonic structures implemented with electronics in a monolithicprocess, and photonic structures implemented with electronics throughhybrid integration such as flip-chip or wire bonding. Embodiments of theinvention are impervious to these details since these variousembodiments all share in common the buried oxide layer beneath thephotonic device layer, the substrate beneath the buried oxide, and theavailability of metal above the photonic device layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-section of an optoelectronic integrated circuitaccording to an embodiment of the invention.

FIG. 2 shows the optoelectronic integrated circuit of FIG. 1 in apackaged photonic optical transceiver.

FIG. 3 illustrates an alternate embodiment of the invention, in which aV-shaped groove is formed in a surface of the photonic chip.

FIG. 4 shows an embodiment of the invention in which an externalwaveguide or fiber is aligned with the direction of the light reflectedby the waveguide grating.

FIG. 5 illustrates the use of a metallic mirror in the embodiment ofFIG. 4.

FIG. 6 illustrates that the optical coupling in embodiment of theinvention is bidirectional.

FIG. 7 shows an embodiment in which lenses are formed in the bottomsurface of the substrate of the photonic chip.

FIG. 8 illustrates that the lens on the bottom surface of the chip maybe comprised of a series of concentric rings.

FIG. 9 shows an embodiment in which mechanical alignment features areformed on the substrate of the photonic chip.

FIG. 10 illustrates a photonic chip having a series of waveguidegratings, and shows a series of external waveguides or fibers adjacent abottom surface of the chip.

FIG. 11 shows a photonic chip having a series of waveguide gratings, andshows a series of external waveguides or fibers adjacent a lateral edgeof the chip.

DETAILED DESCRIPTION

Embodiments of the invention provide an optical coupling structure thatreceives a light beam and couples the beam into a waveguide. In areverse manner, the optical coupling structure may also move light awayfrom the waveguide and out of an optoelectronic IC. The methods offabricating such an optical structure described below implement avariety of conventional semiconductor processes and combinationsthereof, which include: lithography, etching, thin-film deposition, andanti-reflective coatings. Moreover, some of the embodiments also includemethods that may employ conventional wafer to wafer attachment/bondingprocesses.

For simplicity, the description below and related figures describe anoptical coupling structure that includes a silicon-based waveguide thatconsists of single crystalline silicon layer. In alternative embodimentsthe waveguide may be polycrystalline silicon and it may also comprisemultiple layers with specific characteristics for each individual layer(i.e., doping, thickness, resistivity, etc.). The thickness of thewaveguide may be tailored to accommodate one or more modes of apropagating light beam. In addition, although the described embodimentsbelow use silicon-based optical elements, other types of high indexmaterials (i.e., gallium arsenide, lithium niobate, indium phosphide,etc.) may replace silicon-based elements. Further, the waveguide andcoupling regions may take on a variety of shapes and sizes.

FIG. 1 is a cross-section of an optical coupling structure in accordancewith an embodiment of the invention. This embodiment employs asilicon-on-insulator (SOI) wafer 100 (although other high-index-contrastmaterial such as InP, SiN, etc. may be used) in which opticalwaveguides, represented at 102, are patterned into the silicon devicelayer 104. Typical single-mode waveguides are 0.2 um to 0.3 um in heightby 0.4 um to 0.6 um in width. A buried oxide layer 106, typically 1 to 3um thick, exists on the SOI wafer beneath the silicon device layer andabove the silicon substrate layer 110, and a metal layer 112 existsabove the device layer 104.

Both passive (e.g., wavelength-division multiplexers and demultiplexers,waveguide interconnects, power splitters and combiners, etc.) and active(e.g. modulators, detectors, switches, and phase shifters) componentsmay be fabricated in the silicon device layer 104 together withelectronic components used to interface the photonics and for otherprocessing functions. FIG. 1 also shows an external fiber or waveguide114 that includes a connector 116, a turning mirror 120 and one or morepassive mechanical alignment elements 122.

In an embodiment, the substrate material for the photonic chip is Si;photonic circuits have been demonstrated to offer extremely high-densityintegration (nm-scale), CMOS and SiGe bipolar process compatibility, andlow fabrication cost. Optical devices (modulators and detectors) havebeen demonstrated to operate at speeds up to 40 Gb/s and beyond. Inaddition, wavelength division multiplexing (WDM) can bestraightforwardly incorporated into the Si photonic circuits, offeringoff-chip optical coupling of 10× to 100× more channels per fibercompared to parallel implementations. This feature takes full advantageof the integration capability of the Si photonic structures; because oftheir sub-micron dimensions, tens to hundreds of thousands of devicescan be incorporated in a square centimeter, driving the need for dense,highly efficient, and broadband optical coupling schemes.

In the embodiment shown in FIG. 1, a grating 130 is patterned into thephotonic waveguides 102 in the device layer 104 so as to allownear-normal light reflection up/down from the waveguide. The substrate110 is used to enhance performance by focusing the light into/out of thegrating couplers via refractive lenses 132 etched into or bonded ontothe bottom 134 of the substrate layer (back side of the wafer). Theaddition of metal mirrors 136 disposed over the grating structure 130 isused to enhance efficiency by redirecting the upward traveling lightback toward the grating and redirecting this light along the desiredoptical path.

In an embodiment, lens elements 132 are provided on the back surface 134of the substrate 110 aligned on the optical path. An array of lenses areprovided where each lens within the array corresponds to individualwaveguide channels. The lens elements are designed to either collimatethe output light from the photonic chip or to refocus the light into anexternal fiber or waveguide array. In the reverse path, the lenselements accept the light from the external fiber or waveguide array andfocus the light onto the grating coupler which redirects the light intothe photonic waveguides 102. An embodiment of the invention employs lenselements lithographically fabricated on the back side of the wafer.Furthermore, mechanical alignment features 140 can also be fabricated onthe back surface 134 to facilitate passive alignment to the externalfiber or waveguide array 114.

The external fiber or waveguide connector 116 contains an array ofoptical fibers or waveguides arranged approximately parallel to thephotonic chip surface, and also includes turning mirrors 120 to redirectlight normal to the surface and into the fiber array. The fiberconnector may or may not include an array of lens elements, one for eachfiber element. The connector housing 116 can be fabricated from low-costinjection-molded plastic or lithographically patterned using asemiconductor substrate, such as a Si optical bench.

The grating structure 130 may be implemented by etching sub-wavelengthfacets into a fraction of the full silicon layer thickness. For example,the etched areas may then be filled with dielectric material, such thata region of alternating silicon/dielectric facets results. Withoutfurther enhancements, the grating diffracts light in both the upward anddownward directions in substantially equal amounts.

Several methods can be used to improve coupling efficiency in thebackside (through substrate direction). For example, the mirror 136 maybe disposed at a distance of a half-wavelength above the grating 130,where the wavelength includes the effect of the index of refraction ofthe dielectric layer between the grating and the mirror. Alternatively,the mirror 136 may be composed of a dielectric stack of alternatinglow-index and high-index layers.

Another method of enhancing the efficiency is to blaze the grating 130,which is to pattern the facets of the silicon grating such that theorientation of these facets provides substantial reflection in thebackside (through substrate) direction.

An alternate method to reduce the unwanted upward diffracted light is todispose optically absorbing layers above the grating feature. Thismethod does not improve efficiency, but will reduce unwanted scatteredlight which may result in optical crosstalk.

The above discussion describes coupling from the waveguide and output atthe back (substrate) side of the chip. The discussed methods alsoprovide similar efficiency enhancements in the reverse direction—thatis, input from the back of the chip and output into the siliconwaveguide.

An example of a fully packaged optical transceiver module, in accordancewith an embodiment of this invention, is depicted in FIG. 2. Thephotonic chip 100 is flip-chip attached to an organic or ceramic package140. The fiber (or waveguide) connector 116 is assembled onto the backsurface 134 using passive alignment features, for example as discussedabove. A heat spreader 142 is also attached to the back surface of thephotonic chip. The heat spreader also includes mechanical features toaccommodate the fiber array connector.

FIG. 3 illustrates an alternate embodiment. In this embodiment, one ormore V-grooves 160 are fabricated into the rear or second surface 134 ofthe photonic chip and used to reflect light from the diffractivecoupling elements into the substrate in a direction nearly parallel withthe first and second surfaces 162, 134 of the chip to an edge 164 of thechip. This edge of the chip may be made optically smooth by a cleaving,polishing or etching step.

An external fiber or waveguide 170 may be secured to the photonic chipat or adjacent to edge 164 to receive the light reflected by groove 160.This external fiber or waveguide 170 may also be used to direct lightinto the chip 100, toward groove 160, which reflects this light towardgrating 130. As represented at 172, one or more lenses may be disposedat or adjacent the end of fiber or waveguide 170 to focus light into ortransmitted from the fiber or waveguide.

FIGS. 4-11 show additional alternate embodiments. With the embodimentshown in FIG. 4, the external fiber or waveguide 202 is aligned with thedirection of the light reflected by the waveguide grating 130. In thisembodiment, optical coupling is performed via the waveguide gratingcoupler using a refractive substrate with lensed 204 bottom face 134.

With the embodiment illustrated in FIG. 5, optical coupling is enhancedthrough top mirror 136 in the metal stack 112. Optical coupling, as inthe embodiment of FIG. 4, is performed via the waveguide grating coupler130 using a refractive substrate with lensed 204 bottom face. Thisembodiment may also include a polarization diversity scheme viapolarization-splitting waveguide grating and mode-matching to variousfiber dimensions.

FIG. 6 illustrates that the optical coupling is bidirectional—light maybe transmitted from, as well as directed to, the external opticalwaveguide 202. It may be noted, here too, that the embodiment mayinclude a polarization diversity scheme via polarization splittingwaveguide grating and mode-matching to various fiber dimensions.

FIG. 7 shows an embodiment in which the lens 220 on the substrate 110 isformed in the substrate so that the lens does not project beyond thesurface 134 of the substrate.

As illustrated in FIG. 8, the lens 230 on the bottom surface 134 of thesubstrate 110 may be comprised of a series of concentric rings.

FIG. 9 shows an embodiment in which mechanical alignment features 240are formed on substrate 110, around or adjacent lens 220.

FIG. 10 illustrates a photonic chip having a series of waveguidegratings 250, and a series of external waveguides or fibers 252. Each ofthe waveguides or fibers 252 is associated with one of the waveguidegratings 250 and may be used to direct light to or receive light fromthat one grating.

FIG. 11 shows a photonic chip having a V-shaped groove 260, similar tothe groove 160 of FIG. 3, formed in substrate 110 of the chip. As shownin FIG. 11, this photonic chip also includes a series of waveguidegratings 262, and a series of external waveguides or fibers 264 areassociated with these gratings. These external waveguides or fibers arelocated adjacent a lateral edge 164 of the chip, between surfaces 162,134. Each of these external waveguides or fibers 264 are associated withone of the waveguide gratings and may be used to direct light to orreceive light from that grating.

While it is apparent that the invention herein disclosed is wellcalculated to achieve the features discussed above, it will beappreciated that numerous modifications and embodiments may be devisedby those skilled in the art, and it is intended that the appended claimscover all such modifications and embodiments as fall within the truespirit and scope of the present invention.

The invention claimed is:
 1. An optoelectronic integrated circuit forcoupling light to an external optical fiber or waveguide, comprising: asemiconductor body including first and second opposite and parallelexternal surfaces, and further including a metal-dielectric stack, anoptical device layer, a buried oxide layer and a semiconductor substratearranged in series between said first and second surfaces, with theoptical device layer between the semiconductor substrate and the firstsurface of the semiconductor body; at least one optical waveguide formedin the optical device layer for guiding light in a defined plane in theoptical device layer; and a plurality of diffractive coupling elementsdisposed in the optical device layer to couple light from the waveguide,through the buried oxide layer and through the substrate, to the secondexternal surface of the semiconductor body, opposite the first externalsurface thereof, at a near-normal angle to said defined plane to couplethe light from the waveguide to an external optical fiber or waveguide.2. The optoelectronic integrated circuit according to claim 1, furthercomprising an array of lenses formed in the second surface of thesemiconductor body to receive light from or to transmit light to theplurality of diffractive coupling elements.
 3. The optoelectronicintegrated circuit according to claim 1, further comprising a metallicreflective layer formed in said metal-dielectric stack adjacent thediffractive coupling elements to redirect light to the diffractivecoupling elements and therein enhance optical coupling efficiency. 4.The optoelectronic integrated circuit according to claim 1, wherein theplurality of diffractive coupling elements comprise a grating patternedinto the at least one optical waveguide to allow near-normal lightreflection to and from the optical waveguide.
 5. The optoelectronicintegrated circuit according to claim 2, further comprising one or moremechanical elements fabricated on the second surface of thesemiconductor body to facilitate positioning an external optical fiberor optical waveguide on the semiconductor body in a position alignedwith the array of lenses.
 6. An optoelectronic integrated circuit forcoupling light to an external optical fiber or waveguide, comprising: asemiconductor body including first and second opposite and parallelexternal surfaces, and further including a metal-dielectric stack, anoptical device layer, a buried oxide layer, and a semiconductorsubstrate arranged in series between said first and second oppositesurfaces, with the optical device layer between the semiconductorsubstrate and the first surface of the semiconductor body; at least oneoptical waveguide formed in the optical device layer for guiding lightin a defined plane in the optical device layer; a grating patterned intothe optical device layer to couple light from the waveguide to thesecond external surface of the semiconductor body, opposite the firstexternal surface thereof, at a near-normal angle to said defined plane;and an array of lenses formed on the second external surface of thesemiconductor body to receive light from or to direct light to thegrating to couple the light from the waveguide, through the buried oxidelayer and through the substrate, to an external optical fiber orwaveguide.
 7. The optoelectronic integrated circuit according to claim6, further comprising a metallic reflective layer formed in saidmetal-dielectric stack adjacent the grating to redirect light to thegrating and therein enhance optical coupling efficiency.
 8. Theoptoelectronic integrated circuit according to claim 7, for use with anexternal waveguide having a multitude of waveguide channels, and whereeach of the lenses of the lens array receives light in one of saidwaveguide channels.
 9. The optoelectronic integrated circuit accordingto claim 8, wherein the lenses of the array of lenses are refractive,convex lenses lithographically patterned in the second surface of thesemiconductor body.
 10. The optoelectronic circuit according to claim 1,wherein: the second surface of the semiconductor body is a back surfaceof the semiconductor body; the optical device layer, the buried oxidelayer and the semiconductor layer comprise a photonic chip; thediffractive coupling elements provide backside optical coupling from theoptical waveguide fabricated on a front surface of the photonic chipthrough the semiconductor substrate; the diffractive coupling elementscomprise a grating patterned into the optical waveguide; and the gratingcomprises facets etched into the optical device layer and filled withdielectric material, resulting in a region of alternating, siliconfacets and dielectric facets.
 11. The optoelectronic integrated circuitaccording to claim 1, wherein: the first surface of the semiconductorbody is a front surface of the semiconductor body; the second surface ofthe semiconductor body is a back surface of the semiconductor body; theoptical device layer, the buried oxide layer and the semiconductor layercomprise a photonic chip; and the diffractive coupling elements providebackside optical coupling from the optical waveguide fabricated on afront surface of the photonic chip, through the semiconductor substrate,to the back surface of the semiconductor body.
 12. An optoelectronicintegrated circuit for coupling light to an external optical fiber orwaveguide, comprising: a semiconductor body including first and secondopposite and parallel external surfaces, and further including ametal-dielectric stack, an optical device layer, a buried oxide layerand a semiconductor substrate arranged in series between said first andsecond surfaces, with the optical device layer between the semiconductorsubstrate and the first surface of the semiconductor body; at least oneoptical waveguide formed in the optical device layer for guiding lightin a defined plane in the optical device layer; a plurality ofdiffractive coupling elements disposed in the optical device layer tocouple light from the waveguide, through the buried oxide layer andthrough the substrate, to the second external surface of thesemiconductor body, opposite the first external surface thereof, at anear-normal angle to said defined plane; and one or more grooves formedin the second external surface of the semiconductor body to reflectlight from the diffractive coupling elements into a directionsubstantially parallel with the first and second surfaces of thesemiconductor body to an edge of the semiconductor body to couple thelight from the waveguide to an external optical fiber or waveguide. 13.The optoelectronic integrated circuit according to claim 12, wherein atleast one of said one or more grooves is a V-shaped groove extendinginto the semiconductor substrate from the second surface of thesemiconductor body and aligned with the diffractive coupling elements.14. The optoelectronic integrated circuit according to claim 12, furthercomprising a metallic reflective layer formed in said metal-dielectricstack adjacent the diffractive coupling elements to redirect light tothe diffractive coupling elements and therein enhance optical couplingefficiency.
 15. The optoelectronic integrated circuit according to claim1, wherein the plurality of diffractive coupling elements comprise agrating patterned into the optical waveguide to allow near-normal lightreflection to and from the optical waveguide.
 16. The optoelectronicintegrated circuit according to claim 12, further comprising one or moremechanical elements fabricated on the semiconductor body to facilitatepositioning an external optical fiber or optical waveguide on thesemiconductor body in a position aligned to receive the light reflectedby said one or more grooves.
 17. A method of fabricating anoptoelectronic integrated circuit for coupling light to an externaloptical fiber or waveguide, comprising: fabricating a semiconductor bodyincluding first and second and parallel opposite external surfaces, andfurther including a metal-dielectric stack, an optical device layer, aburied oxide layer and a semiconductor substrate arranged in seriesbetween said first and second surfaces, with the optical device layerbetween the semiconductor substrate and the first surface of thesemiconductor body, including forming at least one optical waveguide inthe optical device layer for guiding light in a defined plane in theoptical device layer, and forming a plurality of diffractive couplingelements in the optical device layer to couple light from the waveguide,through the buried oxide layer and through the substrate, to the secondexternal surface of the semiconductor body, opposite the first externalsurface thereof, at a near-normal angle to said defined plane to couplethe light from the waveguide to an external optical fiber or waveguide.18. The method according to claim 17, wherein the fabricating thesemiconductor body further includes forming an array of lenses in thesecond surface of the semiconductor body to receive light from or todirect light to the plurality of diffractive coupling elements.
 19. Themethod according to claim 17, wherein the fabricating the semiconductorbody further includes forming a metallic reflective layer in saidmetal-dielectric stack adjacent the diffractive coupling elements toredirect light to the diffractive coupling elements and therein enhanceoptical coupling efficiency.
 20. The method according to claim 17,wherein the forming the plurality of diffractive coupling elementscomprise patterning a grating into the optical waveguide to allownear-normal light reflection to and from the optical waveguide.
 21. Anoptoelectronic transceiver module comprising for coupling light to anexternal optical fiber or waveguide: a semiconductor body includingfirst and second opposite and parallel external surfaces, and furtherincluding a metal-dielectric stack, an optical device layer, a buriedoxide layer and a semiconductor substrate arranged in series betweensaid first and second surfaces, with the optical device layer betweenthe semiconductor substrate and the first surface of the semiconductorbody; at least one optical waveguide formed in the optical device layerfor guiding light in a defined plane in the optical device layer; aplurality of diffractive coupling elements disposed in the opticaldevice layer to direct light from the waveguide, through the buriedoxide layer and through the substrate, toward the second externalsurface of the semiconductor body, opposite the first external surfacethereof, at a near-normal angle to said defined plane to couple thelight from the waveguide to an external optical fiber or waveguide; afiber array connector assembled on the semiconductor body, andcomprising an array of optical fibers arranged in a plane approximatelyparallel to the defined plane in the optical device layer for receivingsaid light from the waveguide.
 22. The optoelectronic transceiver moduleaccording to claim 21, wherein: the fiber array connector is assembledon the second surface of the semiconductor body; and the fiber arrayconnector further includes an array of lenses for focusing the lightfrom the waveguide onto the fiber array.
 23. The optoelectronictransceiver module according to claim 22, wherein the fiber arrayconnector further includes mechanical alignment elements for positioningthe fiber array connector in a specified position on the semiconductorbody.
 24. The optoelectronic transceiver module according to claim 23,wherein: the semiconductor body further includes an array of lensesformed on the second surface of the semiconductor body to receive lightfrom the plurality of diffractive elements; and the mechanical alignmentelements position the fiber array connector on the semiconductor bodywith the array of lenses of the fiber array connector aligned with thearray of lenses of the semiconductor body.
 25. The optoelectronictransceiver module according to claim 21, wherein: the semiconductorbody further includes at least one groove formed in the second surfaceof the semiconductor body to reflect light from the diffractive couplingelements into a direction substantially parallel with the first andsecond surfaces of the semiconductor body to an edge of thesemiconductor body, said edge being on a third side of the semiconductorbody, between the first and second sides thereof; the fiber arrayconnector is assembled on the semiconductor body with the array ofoptical fibers adjacent said edge of the semiconductor body; and thefiber array connector further comprises one or more mechanical alignmentelements to facilitate positioning the fiber connector assembly adjacentsaid edge of the semiconductor body in a position aligned to receive thelight reflected by said at least one groove.